Parainfluenza viral infection results in serious respiratory tract disease in infants and children. (Tao, et al., 1999, Vaccine 17:1100-08). Infectious parainfluenza viral infections account for approximately 20% of all hospitalizations of pediatric patients suffering from respiratory tract infections worldwide. Id. A vaccine has not yet been approved for the prevention of PIV related disease, nor is there an effective antiviral therapy once disease occurs.
PIV is a member of the paramyxovirus genus of the paramyxovirus family. PIV is made up of two structural modules: (1) an internal ribonucleoprotein core, or nucleocapsid, containing the viral genome, and (2) an outer, roughly spherical lipoprotein envelope. Its genome is a single strand of negative sense RNA, approximately 15,456 nucleotides in length, encoding at least eight polypeptides. These proteins include the nucleocapsid structural protein (NP, NC, or N depending on the genera), the phosphoprotein (P), the matrix protein (M), the fusion glycoprotein (F), the hemagglutinin-neuraminidase glycoprotein (HN), the large polymerase protein (L), and the C and D proteins of unknown function. Id.
The parainfluenza nucleocapsid protein (NP, NC, or N) consists of two domains within each protein unit including an amino-terminal domain, comprising about two-thirds of the molecule, which interacts directly with the RNA, and a carboxyl-terminal domain, which lies on the surface of the assembled nucleocapsid. A hinge is thought to exist at the junction of these two domains thereby imparting some flexibility to this protein (see Fields et al. (ed.), 1991, Fundamental Virology, Second Edition, Raven Press, New York, incorporated by reference herein in its entirety). The matrix protein (M), is apparently involved with viral assembly and interacts with both the viral membrane as well as the nucleocapsid proteins. The phosphoprotein (P), which is subject to phosphorylation, is thought to play a regulatory role in transcription, and may also be involved in methylation, phosphorylation and polyadenylation. The fusion glycoprotein (F) interacts with the viral membrane and is first produced as an inactive precursor, then cleaved post-translationally to produce two disulfide linked polypeptides. The active F protein is also involved in penetration of the parainfluenza virion into host cells by facilitating fusion of the viral envelope with the host cell plasma membrane. Id. The glycoprotein, hemagglutinin-neuraminidase (HN), protrudes from the envelope allowing the virus to contain both hemagglutinin and neuraminidase activities. HN is strongly hydrophobic at its amino terminal which functions to anchor the HN protein into the lipid bilayer. Id Finally, the large polymerase protein (L) plays an important role in both transcription and replication. Id.
In one embodiment, the present invention relates to the construction of a cross-species bovine PIV3/human PIV3 chimeric virus vaccine. Bovine parainfluenza virus was first isolated in 1959 from calves showing signs of shipping fever. It has since been isolated from normal cattle, aborted fetuses, and cattle exhibiting signs of respiratory disease (Breker-Klassen, et al., 1996, Can. J. Vet. Res. 60:228-236). See also Shibuta, 1977, Microbiol. Immunol. 23 (7), 617-628. Human and bovine PIV3 share neutralizing epitopes but show distinct antigenic properties. Significant differences exist between the human and bovine viral strains in the HN protein. In fact, while a bovine strain induces some neutralizing antibodies to hPIV infection, a human strain seems to induce a wider spectrum of neutralizing antibodies against human PIV3 (Klippmark, et al., 1990, J. Gen. Vir. 71: 1577-1580). Thus, it is expected that the bPIV3/hPIV3 chimeric virus vaccine of the present invention will also induce a wider spectrum of neutralizing antibodies against hPIV3 infection while remaining attenuated and safe for human use. Other chimeric parainfluenza virus vaccines are also contemplated by the invention.
The replication of all negative-strand RNA viruses, including PIV, is complicated by the absence of cellular machinery required to replicate RNA. Additionally, the negative-strand genome can not be translated directly into protein, but must first be transcribed into a positive-strand (mRNA) copy. Therefore, upon entry into a host cell, the genomic RNA alone cannot synthesize the required RNA-dependent RNA polymerase. The L, P and N proteins must enter the cell along with the genome on infection.
It is hypothesized that most or all of the viral proteins that transcribe PIV mRNA also carry out their replication. The mechanism that regulates the alternative uses (i.e., transcription or replication) of the same complement of proteins has not been clearly identified but appears to involve the abundance of free forms of one or more of the nucleocapsid proteins. Directly following penetration of the virus, transcription is initiated by the L protein using the negative-sense RNA in the nucleocapsid as a template. Viral RNA synthesis is regulated such that it produces monocistronic mRNAs during transcription.
Following transcription, virus genome replication is the second essential event in infection by negative-strand RNA viruses. As with other negative-strand RNA viruses, virus genome replication in PIV is mediated by virus-specified proteins. The first products of replicative RNA synthesis are complementary copies (i.e., plus-polarity) of PIV genome RNA (cRNA). These plus-stranded copies (anti-genomes) differ from the plus-strand mRNA transcripts in the structure of their termini. Unlike the mRNA transcripts, the anti-genomic cRNAs are not capped and methylated at the 5′ termini, and are not truncated and polyadenylated at the 3′ termini. The cRNAs are coterminal with their negative strand templates and contain all the genetic information in the complementary form. The cRNAs serve as templates for the synthesis of PIV negative-strand viral genomes (vRNAs).
Both the bPIV negative strand genomes (vRNAs) and antigenomes (cRNAs) are encapsidated by nucleocapsid proteins; the only unencapsidated RNA species are virus mRNAs. For bPIV, the cytoplasm is the site of virus RNA replication, just as it is the site for transcription. Assembly of the viral components appears to take place at the host cell plasma membrane and mature virus is released by budding.